Semi-permeable membranes prepared from various synthetic polymeric compositions are being used in an increasing number of commercial and industrial applications. Such uses include such diverse fields as the demineralization of saline water, gas separation, separation of organic materials, and ultrafiltration. Depending on the synthetic polymer used and the intended application, the membranes may be prepared by several methods well known to those skilled in the art. Generally such preparations include the steps of dissolving or dispersing a monomer or a prepolymer into a casting solvent, casting a film from the prepolymer, evaporating at least part of the casting solvent from the cast film, and curing the membrane. See U.S. Pat. No. 3,133,132.
The casting solvents currently in use are mostly organic solvents such as acetone, methyl ethyl ketone, ethyl alcohol, methyl alcohol, methylene chloride, and chloroform. Such solvents are frequently costly, highly flammable or suffer from other drawbacks. Certain casting solvents present health hazards when inhaled as vapors or upon contact with the skin. The discharge of the vapors into the air also may be subject to governmental regulation necessitating some kind of recovery or disposal system. Therefore, it would be desirable to be able to cast a membrane from an aqueous mixture or solution.
Additionally, it is important that the membrane discriminating layer be as thin as possible, while being defect free. Techniques for forming thin, uniform discriminating layers have long been sought.